Differential effects of serine side chain interactions in amyloid formation by islet amyloid polypeptide

Islet amyloid polypeptide (IAPP), a 37 residue polypeptide, is the main protein component of islet amyloid deposits produced in the pancreas in Type 2 diabetes. Human IAPP contains five serine residues at positions 19, 20, 28, 29, and 34. Models of the IAPP amyloid fibril indicate a structure composed of two closely aligned columns of IAPP monomers with each monomer contributing to two intermolecular β-strands. Ser 19 and Ser 20 are in the partially ordered β-turn region, which links the two strands, whereas Ser 28, Ser 29, and Ser 34 are in the core region of the amyloid fibril. Ser 29 is involved in contacts between the two columns of monomers and is the part of the steric zipper interface. We undertook a study of individual serine substitutions with the hydrophobic isostere 2-aminobutyric acid (2-Abu) to examine the site-specific role of serine side chains in IAPP amyloid formation. All five variants formed amyloid. The Ser 19 to 2-Abu mutant accelerates amyloid formation by a factor of 3 to 4, while the Ser 29 to 2-Abu mutation modestly slows the rate of amyloid formation. 2-Abu replacements at the other sites had even smaller effects. The data demonstrate that the cross-column interactions made by residue 29 are not essential for amyloid formation and also show that cross-strand networks of hydrogen-bonded Ser side chains, so called Ser-ladders, are not required for IAPP amyloid formation. The effect of the Ser 19 to 2-Abu mutant suggests that residues in this region are important for amyloid formation by IAPP.